According to the pulse Doppler radar concept, the Doppler shift is used as a means for discriminating moving targets from undesired stationary objects.
Known pulse Doppler radars typically make use of microwave radar signals emitted at a certain pulse repetition frequency (PRF), whereby pulse trains are emitted at regular intervals. By utilising various forms of filtration, for instance using digital filter banks, the radial velocity of the object can be found. For so-called coherent pulse Doppler radars this is accomplished by comparing the phase value of the signal emitted by the radar with the echo signal.
If the main lobe emitted by the radar beam signal is directed against stationary objects, the radar will continuously receive signals or so-called clutter from these stationary objects.
In FIG. 1, which relates to signals obtained from a pulse Doppler radar, borne by moving radar platform F observing object T, the Doppler frequency shifts measured by the radar have been shown. Two peaks, relating to the received Doppler frequency shift of respectively the main lobe clutter, M, and the moving object, T, appear.
The difference between the object Doppler frequency shift and the clutter Doppler frequency shift can be visualised as the distance between the object and the clutter peak.
From the resulting difference, f.sub.d, between the Doppler frequency shift relating respectively to the main lobe clutter, M, and the moving object, T, the radial speed, v.sub.r, of the object can be calculated according to the relation: ##EQU1##
where .lambda. is the microwave wavelength. The radial speed, v.sub.r, of the given object can be expressed as v.sub.r =v.multidot.cos(.theta.), where v is the object speed, and .theta. is the angle made by the object trajectory and the line joining radar and object.
It is noted that the above difference in frequencies and hence the radial speed measurement is independent of the speed of the radar platform.
When using a pulse Doppler radar on an aircraft this has the advantage that a moving object can be detected although the object is close to the ground or other stationary objects as seen from the radar. Consequently, the moving object can be detected with high reliability. This also applies to stationary radar platforms.
As is well known, the task of a typical radar system is normally not only restricted to detecting certain objects, but it should also provide for the detected objects position, speed and heading.
For pulse Doppler radars, the time it takes for a pulse train to travel to an object and back to the radar follows the relation: ##EQU2##
where R is the distance to the object, c is the speed of light and t is the period between an emitted pulse and the associated received pulse.
By measuring the time, t, it should be possible to find the distance to the object. However, since the pulse repetition frequency, PRF, typically is high for many radar systems, it is difficult to identify the echoes. Consequently, the distance to the object can in many instances not be resolved unambiguously.
In these cases, the range is resolved by means of other or by additional methods.
A particular pulse Doppler radar technique is the MPD (Medium PRF Pulse Doppler) mode, which operates in the range of 3-30 kHz PRF emitting microwave signals in the range of 1-10 cm.
By using so-called PRF switching, by which the object is illuminated with plural PRF's, the distance to the object and/or the radial speed of the object can be unambiguously resolved. However, this technique still leaves something to be desired, when it comes to the accuracy of the range information.
Another technique is the HPD (High PRF Pulse Doppler) mode in which the PRF is chosen so high that the objects radial speed can be obtained.
However, in order to provide an unambiguous determination of the distance to the object, it is necessary to modulate the pulse train. Unfortunately, this has the effect that the radar search performance is somewhat degraded.
Moreover, when using modulation, the accuracy of range information is not entirely satisfactory. Hence, this method leads in many radar systems to inaccurate position measurements.
According to well-known trigonometric methods, which are utilised in known radar systems, the position of an object can be determined by taking a bearing to the object and combining this measurement with a measurement of the objects range. The velocity vector of the moving object can be found from performing position measurements at different points in time.
Alternatively, conventional cross-bearings can be performed from dispersed radar units.
The accuracy of the resulting data will depend on the precision of the angular measurement and the range measurement. The position measurements will often be of limited accuracy. This in turn will result in poor accuracy of the calculated velocity vector or demand a long measuring time.
Moreover, if moving objects are close to one another and substantially have the same course and speed, there is a risk that these objects might not be distinguished from each other and that the velocity vector for any of the adjacent objects may be calculated erroneously.
In FIG. 2, this situation has been illustrated. If radar unit F1 measures the position of object T1 for calculating the course and speed of T1, there is a risk that radar unit F2 will falsely identify object T2 as T1. Consequently, the calculation of T1's velocity, corresponding to the dotted line, will be erroneous. The correct velocity of T1 is represented by the solid line.